Take a look at this discussion here Home Networking
The difference between speed test and another is pretty profound. It works like this (not very technical and skips over a tonne of technical aspects, but it demonstrates the issue at hand).
If you imagine 2 users, call them Bob and Mike.
There is 100Mbps split between the two of them at the local exchange, node or street box.
Bob downloads a file (1 threaded), he gets 100Mbps.
Now Mike runs a speedtest, single threaded. Bobs download slows to 50Mbps, Mikes speedtest gets 50Mbps (sharing available backhaul).
Say Mike runs a speedtest with 9 threads, Bobs 1 threaded download falls to 10Mbps, Mikes speedtest shows 90Mbps.
With multi threaded speedtests, congestion is hidden a lot more.
Hence ISPs send you to www.speedtest.net as it's multi threaded and very good at covering up congestion issues.
Your ISP will try to use the multi-thread test as the standard against which they test any claim that you are not getting the download you pay for.
Congestion in your local network could easily affect the outcome of your single-thread test depending on the architecture of your network. If you are lucky you will have a single coax or a twisted pair copper line all to yourself. straight from your local street box. This would give you no local congestion, but the same issues as anybody else when negotiating the wider network, much of which has nothing to do with your ISP. If you are a cable user, more likely you are sharing a coax with other users giving you local congestion on top of everything else.
To test your speed you might also use a really fast file download site, e.g Visual studio Try two or three files at the same time then add the download speed of them all to get the maximum. ( Windows show speeds using MB rather than Mb. Just multiply the MB result by 8 to give Mb. )
When fighting this out with your ISP they always have the get out that the download speed they quote to you when you buy in to their contract is an 'up to' speed. e.g. 'Up to 200Mb'. If you look at their small print they will cite congested times of day as possible causes of slow test results. They can also suggest that your home kit might be causing problems.
The good thing is that unless you are experiencing poor service e.g. problems with browsing, streaming, game play or downloading and up loading large files then you probably have nothing to complain about. Speed tests are not a good indicator of the service you are getting, but your daily experience as a user is.
Only yesterday I was having trouble streaming from my own cloud (Livedrive) . I rang my ISP (Virgin Media - UK) and explained that even though my speed test gave me my usual >300Mb download speed and 20Mb up speed, my videos experiencing showing slow buffering. They tested my connect right down to my modem and found that power levels were too low on some of my channels. They adjusted this and put it right.
My speedtest.net result